This invention relates to sedimentation systems used to separate liquid and solid components of a feed slurry and more specifically relates to feedwell apparatus employed in thickener/clarifier tanks.
Thickener/clarifier tanks are used in a wide variety of industries to separate feed slurry comprising a solids or particulate-containing fluid to produce a “clarified” liquid phase having a lower concentration of solids than the feed slurry and an underflow stream having a higher concentration of solids than the feed slurry. Thickener/clarifier tanks conventionally comprise a settling tank having a floor and a continuous wall, which define a volume within which the clarification process takes place. Thickener/clarifier tanks also include an influent feed pipe for delivering an influent feed stream to the tank, an underflow outlet for removing settled solids from the tank, and a fluid discharge outlet for directing clarified liquid away from the tank. Thickener/clarifier tanks may also include a rake assembly having rake arms for sweeping along the floor of the tank, and may include an overflow launder or bustle pipe for collecting clarified liquid near the top of the tank.
Thickener/clarifier tanks of the type described operate by introducing an influent feed stream into the volume of the tank where the influent is retained for a period long enough to permit the solids to settle out by gravity from the fluid. The solids that settle to the bottom of the tank produce a sludge bed near the bottom of the tank, which is removed through the underflow outlet. Clarified liquid is formed at or near the top of the thickener/clarifier tank and is directed away from the tank for further processing or disposal. Settling of solids may be enhanced in some applications by the addition of a flocculant or polymer that forms agglomerates that settle more rapidly. In many applications, an objective of fluid clarification is to enhance the settling process to achieve a high throughput of solids, and thereby enhance solids recovery.
Many thickener/clarifier tanks are constructed with a feedwell, usually centrally located within the tank, into which the influent feed stream is delivered. The feedwell generally serves the purpose of reducing the fluid velocity of the incoming influent feed stream so that the energy in the stream may be dissipated to some degree before entering the tank. Dissipation of energy in the influent feed stream lessens the disruptive effect that the incoming influent feed has on the settling rate of the solids in the tank. In other words, introduction into a thickener/clarifier of an influent feed stream under high fluid velocity tends to cause turbulence in the tank and compromises the settling rate of solids. A feedwell may be structured in a variety of ways, therefore, to create or enhance dissipation of energy in the influent feed. See, e.g., U.S. Pat. No. 3,006,474 to Fitch, and U.S. Pat. Pub. No. 2009/0173701 to Egan, III.
Fluid flow analysis of conventional feedwells suggests that there are areas of high fluid velocity present where an influent feed stream tangentially intercepts and disrupts the constrained vertical fluid flow within central portions of the feedwell. Consequently, localized high shear rates and flow non-uniformities are found in these areas. Such high shear rates and non-uniformities generally create uneven distributions of mixture discharging from the feedwell, particularly as the diameter of the feedwell increases and the aspect ratio of the feedwell changes. These problems may be attributed to discrete feeding of influent streams through one or more localized entrances, where tangential feedpipes project streams that sharply disrupt the constrained vortex within the feedwell.
Several attempts have been proposed to improve the distribution of flow within conventional feedwells. For example, obstructing elements and orifices have been provided in order to promote a better distribution due to the effect of friction or pressure drops associated with the boundary layers and high shear rates. However, such solutions rely on friction, and therefore, may require extra pumping power or fluid potential energy to overcome frictional losses. Moreover, such solutions have limited ranges of operability.
FIGS. 20-23, 26, and 28 illustrate some of the problems associated with conventional tangential inlet feedwells. FIG. 20 shows a thickener/clarifier tank comprising a conventional tangential inlet feedwell having a circular or cylindrical shape. A sludge raking structure 10 is supported for rotation upon a center pier 11, or from a bridge drive (not shown). A drive mechanism 12 of any suitable known construction is mounted atop the pier, or from a bridge, providing the driving torque for the rake structure 10. In this particular embodiment, the pier 11 also supports the inner end of an access bridge 13, while some thickener mechanisms are bridge mounted.
Rake structure 10 comprises a central vertical cage 14 surrounding the pier, and rake arms of girder-like construction extending rigidly from the cage. Rake structure 10 has one pair of long rake arms 15, 16 opposite to one another, and, if required, a pair of short rake arms 17, 18 disposed at right angles thereto, all arms having sludge impelling or conveying blades 19 fixed to the underside thereof.
Rake structure 10 operates in a settling tank 20 to which a feed suspension, feed pulp, or slurry stream 2060 is supplied through feed pipe or infeed conduit 21. Infeed conduit 21 terminates in a feedwell 2040 having a cylindrical body 2042 which surrounds the top end portion of the rake structure 10 and is supported by pier 11.
Tank 20 may be of usual construction, comprising a bottom 24 of shallow inverted conical inclination, and formed with an annular sump 25 around the pier, to which settled solids or sludge are conveyed by rake structure 10. Scraper blades 26, unitary with rake structure 10 and substantially conforming to the profile of sump 25, move the collected sludge to a point of delivery from the sump, as by way of a discharge pipe 27.
Infeed conduit 21 is generally connected upstream of feedwell 2040, although the infeed conduit 21 could simply extend to or over the feedwell 2040 to deliver the slurry stream 2060 thereto. Feedwell 2040 has an annular shelf 2049 (FIG. 21) with an inner edge 2047 defining a circular discharge opening 2048 and a circular outer edge 2045 contiguous with a cylindrical sidewall 2042 of the feedwell. Infeed conduit 21 is connected to the feedwell 2040 via a feedwell inlet 2041 so as to deliver slurry stream 2060 tangentially to a circular path inside the feedwell along the cylindrical sidewall 2042. Infeed conduit 21 or feedwell inlet 2041 may incorporate an eductor structure including a nozzle extending into an open or closed channel for diluting the slurry stream 2060 with clarified liquid from the surrounding thickener/clarifier settling tank 20, via a momentum transfer or eduction process (see for example, U.S. Pat. Nos. 5,893,970 and 5,389,250).
Turning now to FIG. 22a, vertical discharge velocities for a conventional tangential inlet feedwell 2040 having a cylindrical sidewall 2042 are shown. As slurry stream 2060 moves through inlet 2041, past a point of intersection 2043, and along the annular shelf 2049 and cylindrical sidewall 2042, fluid passes through discharge opening 2048 and non-uniformly discharges into tank 20. In the particular embodiment shown, computational fluid dynamic (CFD) analysis was performed assuming a medium-sized feedwell approximately 6 meters in diameter with an inlet flow velocity of approximately 1.8 m/s, a settling velocity of approximately 20 m/h, and approximately 12% by weight flocculated solids in water, wherein the flocculated solids are approximately 2 mm in diameter. Vertical flow velocities are seen to be highest during the first 90 degrees of travel around the feedwell 2040. As shown, a crescent-shaped area 2102 of infeed discharges strongly downward at approximately 1.0-1.5 m/s into settling tank 20, adjacent the inner edge 2047 of discharge opening 2048. Such high velocities may cause flocculated particle breakdown, disrupt sediment resting at the bottom 24 of settling tank 20, or unevenly distribute flocculated particles circumferentially around the tank 20 which may lead to a decrease in overall efficiency of the thickener/clarifier and potentially overload the rake drive mechanism 12. A second non-uniform annular band 2104 of fluid located radially inwardly of area 2102 and inner edge 2047 discharges at a slightly lesser downward velocity of approximately 0.5-1.0 m/s into settling tank 20. A third non-uniform annular band 2106 of fluid located radially inwardly of area 2104 discharges downward into the settling tank 20 at an even lesser rate, between 0.5 and zero m/s. A large central region 2108 within a majority of opening 2048 occupies fluid that moves slowly upward, away from the bottom 24 of tank 20 with velocities up to 0.5 m/s.
FIG. 22b shows velocity vectors 2010 exiting a bottom portion of the feedwell 2040 adjacent inner edge 2047. As shown, an area 2074 of higher fluid velocity discharge is apparent during the first 180 degrees of vertical flow, and an area 2072 of lesser fluid velocity discharge is apparent during the latter 180 degrees of vertical flow. FIG. 22c shows a region of increased acceleration 2082 as the influent stream passes from inlet 2041 into the feedwell 2040, and an increased fluid velocity zone 2084 adjacent the inner edge 2047 during the first 90 degrees of vertical flow. FIG. 22d further shows region of increased acceleration 2082 and increased fluid velocity zone 2084 shown in FIG. 22c. 
Turning now to FIG. 23a, attempts have been made to “deflect” strong initial downward fluid velocities in tangential inlet feedwells similar to the one exemplified in FIGS. 22a-c by incorporating a chord structure 2144 spanning two points along the inner edge 2147 of the annular shelf 2149. However, as shown, CFD analysis suggests that as slurry stream 2160 moves through inlet 2141, past intersection 2143, and along both the annular shelf 2149 and cylindrical sidewall 2142, fluid passes through circular opening 2148 and discharges into the tank 20 non-uniformly. Moreover, the structure 2144 reduces both the perimeter and area of discharge opening 2148, and increases the number of localized fluid accelerations. In the particular embodiment shown in FIG. 23a, CFD analysis was again performed assuming a medium-sized feedwell approximately 6 meters in diameter with an inlet flow velocity of approximately 1.8 m/s, a settling velocity of approximately 20 m/h, and approximately 12% by weight solids in water, wherein the flocculated solids are approximately 2 mm in diameter. Vertical velocities are shown to be highest in areas 2102, 2104 adjacent the first 90 degrees of travel around the feedwell 2140 past the chord structure 2144, and also in areas 2102, 2104 adjacent corners defined between the straight inner edge 2147b of the chord structure 2144 and the circular inner edge 2147a of opening 2148. In areas 2102, fluid discharges strongly downward at approximately 1.0-1.5 m/s into settling tank 20. Such velocities may cause flocculated particle breakdown, disrupt sediment resting at the bottom 24 of settling tank 20, or unevenly distribute flocculated particles circumferentially around the tank 20, which may reduce the efficiency of the thickener/clarifier. In areas 2104, fluid discharges into settling tank 20 at a slightly lesser downward velocity of approximately 0.5-1.0 m/s. In area 2106, fluid discharges downward into the settling tank 20 at an even lesser rate, between 0.5 and zero m/s. A large central region 2108 occupying a majority of opening 2148 contains fluid that may be static or may move slightly upward, away from the bottom 24 of tank 20 at velocities up to 0.5 m/s.
FIGS. 26 and 28 show non-uniform radial flows associated with conventional tangential inlet feedwells 2040, 2140 having cylindrical sidewalls 2042. The figures shown are time-lapse photos from scaled down dye-tests taken at 5 second, 10 second, and 20 second intervals, respectively, from left to right. As shown in FIG. 26, at a flow rate of approximately 0.04 cubic meters per hour, a majority of discharge 70 generally moves toward and settles in one quadrant of a settling tank 20. Similarly, as shown in FIG. 28, at a flow rate of approximately 0.09 cubic meters per hour, a majority of discharge 70 generally stays contained within only about half of the settling tank 20. This uneven distribution of discharge may reduce settling times and a decrease in overall efficiency.